Method for gasification of subterranean carbonaceous deposits



Nov. 23, 1954 F, PEARCE ETAL 2,695,153

METHOD FOR GASIFICATION OF SUBTERRANEAN CARBONACEOUS DEPOSITS Filed Dec. 9, 1950 2 Sheets-$heet 1 STEAM a OXYGEN STEAM a OXYGEN PRODUCT OXYGEN GAS PROCESSING INVENTORS FRANK G. PEARCE JAMES E. LATTA BYMQVAJ ATTORNEY Nov. 23, 1954 F. G. PEARCE r-rrAl. 2,695,153

METHOD FOR GASIFICATION OF SUBTERRANEAN CARBONACEOUS DEPOSITS Filed Dec. 9, 1950 2 Sheets-Sheet 2 TO COOLING STATION FIG. 2

INVENTORS FRANK G. PEARCE JAMES E. LATTA BYMhu/47 ATTORNEY United States Patent METHOD FOR GASIFICATION 0F SUBTERRA- NEAN CARBONACEOUS DEPOSITS Frank G. Pearce and James E. Latta, Tulsa, Okla., as-

signors to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application December 9, 1950, Serial No. 200,026

7 Claims. (Cl. 262-6) The present invention relates to the underground gasification of carbonaceous deposits and to a specific method by which such an operation can be economically effected. More particularly, it pertains to a method for facilitating uniform combustion of such deposits to obtain synthesis and fuel gas and to an economical system for gathering such gas and transporting it to a permanently located processing plant.

Prior methods for the underground gasification of carbonaceous materials, such as coal, contemplated effecting such operations in either (1) abandoned mines which had previously been worked to the point where further removal of coal to the surface by means of machinery and manpower was uneconomical or (2) specially formed entries dug into surface coal outcroppings. Entries of this type may be of substantial size, i. e., 6 to 8 feet wide, 3 to 4 feet high, and from about 150 to about 250 feet long. After combustion was initiated, gaseous products were withdrawn from the combustion zone to the surface through a separate drill hole or gas withdrawal well which was in communication with an entry leading to the burning area. Combustion was supported by injecting air into the entry and over the burning coal. While the conversion of coal in this manner to useful gaseous products eliminates the relatively high costs associated with the removal thereof to the surface, such method requires the utilization of a complex system of entries which are both expensive and hazardous to create.

Accordingly, it is an object of our invention to provide an improved and economical means for the gasification of subterranean carbonaceous deposits. It is a further object of our invention to provide a method for effecting underground gasification of such carbonaceous deposits in the absence of the relatively complex and expensive system of entries previously employed. It is also an object of our invention to provide a method for the gasification of subterranean carbonaceous deposits whereby oxygen-containing gases may be supplied to the zone of combustion through artificially formed channels or fractures in the formation. It is a still further object of our invention to provide a pattern of injection and withdrawal wells extending into a coal seam wherein said wells are connected to one another by means of fractures in said seam whereby continuous passage of product gas formed between the injection wells and the gas withdrawal wells may be maintained, said gas forced through the fractures into the gas withdrawal wells and thereafter transported to the surface where the product gas is collected, cooled and sent to a permanently located process plant for further treatment.

In accordance with our invention, a gas process plant, i. e., a hydrocarbon synthesis or fuel gas plant, is preferably centrally located on the surface of a field having underlying carbonaceous deposits. Surrounding the aforesaid centrally located plant is a group of product gas withdrawal wells connected to a suitable gathering system which serves to collect and transport the hot product gas to a series of separate cooling plants. The proximity of the gas withdrawal wells to the processing plant is determined largely by the amount of underground support considered necessary to safely sustain the plant. In this connection, the quantity of subterranean support required by the gas processing plant will be hereinafter referred to as the supporting area. At such plants, the cooled gases may, if desired, be compressed, after which they are sent on to the main processing plant. Located beyond the above-mentioned gas withdrawal wells and also preferably surrounding the processing plant is a second series of wells into which can be injected air, oxygen or mixtures of steam, air and/or oxygen. These wells are all connected by a suitable manifold system into which the gasifying stream is fed from the processing plant and the various cooling stations. Channels suitable for providing an adequate surface for the reaction and for transmitting components of the product gas to the gas withdrawal wells are created in accordance with a number of methods such as, for example, setting off explosives within an injection Well thereby fracturing the formation so as to produce intercommunicating, gas-permeable passageways between said withdrawal and injection wells. Other procedures for forming such passageways include a method whereby the formation between the two types of wells is' hydraulically fractured. After the necessary channels have been formed between the injection and product gas Withdrawal wells, the temperature at the bottom of the injection wells is brought to the kindling point by means of a Thermit bomb or other suitable device. Combustion is initiated and maintained by feeding an oxygen-bearing gas down the injection wells and into the burning area, usually in the presence of steam. The resulting gaseous products of combustion, principally carbon dioxide and water, then pass into previously created channels or passageways where they come in contact with free carbon and are converted into the desired product gas which consists chiefly of carbon monoxide and hydrogen. The gas thus formed is conducted into the withdrawal wells via the aforesaid intercommunicating channels collected by the withdrawal well gathering system and thereafter sent to one or more cooling units. such gas is reduced to the desired level, it is sent to the processing plant where it is purified, after which it may be employed as fuel gas or as a suitable feed for the synthesis of hydrocarbons.

Our invention may be further illustrated by reference to the accompanying drawings in which Fig. 1 is a detailed diagram of a preferred scheme for working an area having extensive underlying carbonaceous deposits. Fig. 2 is a cross-sectional view, diagrammatically presented, of an injection well and a product gas withdrawal well extending into a carbonaceous deposit such as, for example, coal, and illustrating a ground plan of injection and withdrawal wells connected by channels created by fracturing the coal seam.

Referring to Fig. 1, gas processing plant 2 comprising gas purification units, synthesis and/or fuel gas plants and an oxygen plant is located preferably centrally to a series of gas withdrawal wells 4, spaced from about 50 to about 200 feet apart. These wells surround gas processing plant 2. Oxygen, preferably of about per cent purity, is fed from lines 6 and combined with steam from lines 11 generated at cooling stations 8. Thereafter, the resulting mixture is introduced into circular distribution system 10 through line 12; Since the quantity of-steam generated at the cooling units is considerable, there is a sufficient supply for use in preparation of a suitable gasification stream. Any remaining steam may be employed for the compression of low-pressure product gas or for plant use. The gasification mixture of steam and oxygen is forced into a series of injection wells 14, located in a circular pattern and concentric with the gas withdrawal wells surrounding gas processing plant 2. As gasification of the coal proceeds, the coal adjacent to the gas injection wells is consumed, and the combustion face or flame front moves toward the product gas withdrawal wells. This forward movement should be stopped at a point sufficiently removed from the product gas withdrawal wells to allow substantially complete reversion of the initial products of combustion to carbon monoxide and hydrogen. The product gas thus produced, which is at a temperature in the neighborhood of 1200 to 1400 F., is gathered by lines 16 and transported to cooling stations 8, where it is cooled to the desired temperature, usually about F. The resulting gasmay then be employed as fuel or, if desired, it may be used in the synthesis of hydrocarbons by contacting a fluidized bed of a suitable iron catalyst under known, synthesis conditions. Also, in the event gasification After the temperature of j is effected in a relatively shallow coal scam, the gas pressure will not be suificient and should be compressed to about 300 to 400 p. s. i. g. before it is transferred through lines 18 to the process plant. As the product gas is produced by contact of the primary combustion products with the carbon on the surfaces of previously created channels, it is forced toward withdrawal wells 4 through channels or fractures produced in accordance with the procedure which is explained more fully below. After the combustion face has advanced so that the quantity of the gas being produced is no longer satisfactory, the injection gas is switched to a series of wells located still farther from plant 2 but grouped in a circular. pattern as defined by line 20. This new group of wells is drilled and connected with the existing injection wells, preferably by means of fracturing the formation between the two groups of wells. After the necessary intercomniunicating channels have thus been established, combustion is initiated in the new injection wells, and the flame front again moves toward the ring of withdrawal wells. When the initial group of injection wells is reached, the point of injecting the gasification stream should again be changed to a new perimeter located still farther away from the main gas processing plant. Since, with reasonable fiow of the gasitication stream the aforesaid intercommunicating fractures formed between the two sets of injection wells cause excessive pressure differential, a portion of the gasifying material is continuously injected at the next removed series of wells prior to initiation of combustion in the latter. This is generally considered necessary to increase the width of the fractures or the number thereof in the preparation for full flow of the gasitication streamyhence, development to this extent of the area to be gasified should always be at least one stage ahead of the current operation. In this manner, the gas withdrawal wells, cooling stations and processing plant, all of which have a relatively high initial cost, may always be maintained and operated at the same locations. Underground gasification of coal or other carbonaceous matter continues as a routine operation comprising drilling injection wells and fracturing the formation, preferably in concentric rings at increasing distances from the main plant site.

Operation of the process of our invention is illustrated in further detail by Fig. 2, in which channels 22 caused by fracturing coal seam 24 connect injection well 26 with product gas withdrawal well 28. Casing 30 in each of the wells extends into the coal seam and is held in place by cement or other suitable material 32 Qoal adjacent injection well 26 is ignited, and combustion is maintained by injecting air into the zone of combust on through pipe 34 by means of pump 36. As combustion proceeds the product gas thus formed is forced through channels 22 into the lower portion of product gas withdrawal well 28, after which the gas is removed therefrom through pipe 38 and sent to a cooling station. In setting the casing into the drill hole, the former should generally extend into the coal seam for a distance corresponding to approximately one-fifth to one-fourth of the thickness of the seam; and the drill hole, per se, should generally extend farther into the seam for a distance of about 12 to 18 inches from the bottom of the coal deposit. Ordinarily, it is preferable for the bottom of the drill hole to lie within the coal seam. Its extension into the clay bed below the seam is usually to be avoided inasmuch as the fracturing method employed should be confined within the scam in order that satisfactory fracturing pressures can be reached.

One of the most important variables in the gasification of coal is the ratio of steam to oxygen employed in the gasification stream. In general, we have found that maximum utilization of oxygen can be achieved at steam tooxygen ratios of from about 1.5 to 3.0 (mol volume basis). However, steam-to-oxygen ratios as high as 6:1 may be employed. Generally, combustion of coal or other carbonaceous material between a given group of in ection wells and the gas withdrawal wells may be continued until the combined quantity of carbon monoxide and hydrogen present in the product gas falls below about 70 dr basis ln ciniieci ion wi th the operation here involved, it is to be pointed out that the fir t rea tion f oxygen or the oxygen-containing "gasification stream w 1th a carbon surface results in the formation of carbon dioxide. Wa-

ter is produced simultaneously from the hydrogen-containing constituents. These reactions are rapid and high ly exothermic and cause a very substantial temperature rise at the combustion zone. After the initial reaction, the resulting carbon dioxide and water react with coke and coat volatilization products to form carbon monoxide and hydrogen. Since these latter reactions are endothermic, the temperature decreases as reaction progresses until a state is ultimately reached at which reaction ceases because of equilibrium considerations. The latter reactions are relatively slow, and sufiicient contact time should be allowed to insure that a large proportion of the initially formed carbon dioxide and water are converted to carbon monoxide and hydrogen. if no steam were admitted with the oxygen, the initial combustion temperature should be extremeiy high, for the entire heat release would appear as sensible heat of the combustion products. The addition of any diluent, such as nitrogen or steam, decreases the combustion temperature; and the amount of such diluent introduced can be adjusted to maintain the temperature at any desired value above that necessary for combustion to continue. The maximum yield of carbon monoxide per mol of oxygen with no steam addition is equal to two rnols of carbon monoxide plus the hydrogen associated with two mols of carbon in the coal. When these conditions are achieved, the product gas is extremely hot (approximately 3,000 E), and additional reaction with coal can be obtained by adding water vapor to the product gas. Thus, the addition of Water vapor to oxygen can, in some instances, increase the yield of synthesis gas per unit of oxygen fed. A point is reached, however, where increased steam dilution causes decreased oxygen efficiency. This results from a severe lowering of the combustion temperature and subsequent incomplete reaction of carbon dioxide and water with carbon to produce carbon monoxide and hydrogen. As a result of these opposed efiects, we have found that the optimum steam-to-oxygen ratio lies between about 1.5 and about 3.0. The gasitication stream prior to injection is preferably under sutficient pressure such that the pressure of the resulting product gas from the gas withdrawal wells is of the order of that required for its intended use. For example, if the resulting gas is to be employed in the synthesis of hydrocarbons, it is preferably withdrawn at pressures of the order of from about 250 to about 450 p. s. i. g.

The melting points of ash and strata adjacent to coal may be about 2,000 to 2,500 F. The initial combustion temperature may be controlled to lie within that range, but efficiency is low and operating costs very high inasmuch as an eight-to-one steam-to-oxygen ratio is required to regulate the maximum possible reaction temperature at 2,300 P. Therefore, for any economically attractive process, the steam-to-oxygen ratio should be considerably lower; however, conditions allowing ash fusion and fusion of adjacent strata still may exist at the combustion face. Although temperatures above the fusion point prevail, satisfactory operation is possible and is supported by reports in which Russian investigators indicated that successful underground gasification projects have been effected using air instead of mixtures of steam and oxygen since the use of air results in combustion temperatures higher than the melting point of the surrounding formations. Fusing conditions can be tolerated provided the fused material does not form to such an extent that the gasification channels become plugged. Since the rate of rock fusion is dependent upon the combustion zone temperature, procedures employing a tWo-to-one steam-tooxygen ratio result in a higher rate of fusion. This does not necessarily mean, however, that such a procedure is less likely to represent an operable condition than situations in which higher steam-to-oxygen ratios prevail in the gasification stream. For example, the increased fluidity of the slag thus produced at 6,000 F. may prevent the formation of a froth; and, although greater in weight, the slag may have a lower volume after it solidifies.

The technique employed in fracturing coal deposits in carrying out the process of our invention may, in general, be effected in accordance with the methods disclosed in U. S. 2,596,843 granted May 13, 1952, to R. F. Farris and U. S. 2,596,845 granted May 13, 1952, to J. B. Clark. The procedure there described may be applied to the problem of underground coal gasification and involves injecting into the drill hole a liquid which is substantially incapable of penetrating the coal seam, hereinafter-referred to as a low penetrating liquid. Ordinarily, the viscosity of such a liquid is not substantially less than about 30 centipoises nor greater than 2 to 5 poises. This liquid is forced into a confined zone at the lower portion of the drill hole, i. e., the portion of the drill hole which extends into the coal seam, and thereafter fractures or channels are produced in the latter by the application of hydrostatic pressure. The low penetrating fluid present in the channel thus formed impairs the permeability thereof and may be either burned away during the subsequent gasification operation or may be removed from the fracture by pumping in a suitable solvent therefor at the formation overburden pressure. Satisfactory removal of the fracturing or low penetrating fluid may likewise be accomplished by adding thereto, prior to injection into the drill hole, a substance capable of substantially reducing the viscosity of the fluid after fracture of the seam has been effected.

It is known that when a fracture is created in a relatively impermeable zone and such opening is then closed, the permeability of the zone is noticeably increased. This permeability may be further increased by mixing into the low penetrating fluid granular propping or spacing agents such as, for example, and, to hold open to a greater degree the channels thus made. In fracturing coal seams in accordance with our invention, we prefer to incorporate the propping agent in the low penetrating fluid to the extent of from about 30 to about 60 weight per cent of the final mixture.

A low penetrating fluid can be prepared by adding from about 3 to about per cent of a bodying agent, such as a fatty acid salt, for example, an aluminum soap, to a hydrocarbon fluid, such as crude or refined oil. Other bodying agents may comprise. a colloid material, a high molecular weight olefin polymer, particularly high molecular weight linear polymers, such as polypropylene and oil or water-soluble plastering agents, such as blown asphalt or pitch.

Thus, a typical low penetrating fluid is prepared by adding soap, ordinarily in a concentration of about 6 weight per cent, to a measured volume of crude oil, in the presence of agitation. A flow type mixer for the soap and crude oil may be used at the drill hole head with soap which reacts readily to produce a gel in the oil. It is found that the gel will develop in the drill hole. Therefore, since the more viscous gels are difficult to pump, the soap-liquid dispersion may be introduced into the hole before maximum viscosity is reached. Accordingly, when gellation has proceeded to a point at which the viscosity is sufficient to maintain substantially all the parts of the undispersed soap and granular propping agent in suspension, the dispersion is pumped into the drill hole. The gel may be injected immediately into the coal seam or it may be allowed to stand in the hole until the maximum gellation has developed, as indicated by a sample retained at the surface. By this means, highlyviscous liquids are available for producing the fracture where such liquids could not normally be pumped.

The low penetrating fluid prepared as described above is then introduced into the drill hole through a tubing, the lower end of which extends into the coal seam. A packer is preferably employed to isolate and confine the fluid within the seam to be fractured. Pressure is applied to the low penetrating fluid either directly or indirectly by pumping another fluid into the hole on top of the low penetrating fluid thereby building up a hydrostatic pressure at the coal seam sufiiciently great to cause the latter to part or fracture. The order of magnitude of pressure required to part or fracture a formation (hereinafter referred to as the formation breakdown pressure) is roughly equivalent in pounds per square inch to the depth of the coal seam in feet. This pressure varies, however, from place to place, depending upon the depth and the nature of the formations, folding of the formations, and the like. The pressure required to fracture a coal seam is, in general, equal to the sum of the pressure required to overcome the bonding strength of the coal and the pressure required to lift the effective overburden which is normally much less than the bearing pressure of the overlying formation.

In any event, it is necessary to apply at the face of the coal seam a pressure equal to or greater than the formation breakdown pressure, and such a pressure is readily recognized. In this connection, when the fracturing fluid is forced against the coal seam as above described, pressure builds up as fluid is pumped into the drill hole.

Eventually, the pressure levels off or drops rather abruptly in an amount substantially equal to the pressure required to overcome the bonding strength of the coal and thereafter assumes a substantially constant value. This pressure maximum which is reached just prior to the abrupt drop in pressure is regarded as the formation breakdown pressure, and a pressure history such as described indicates formation fracturing has been obtained.

After a satisfactory rupture of the coal seam has been effected, a solvent for the low penetrating fluid may be injected into the fracture at substantially the formation overburden pressure, since the fracture has already been produced, thereby tending to follow the crevice or channel formed by thelow penetrating fluid. This solvent may be injected into the coal seam in any amount, but normally a quantity between about one-half to about five times, preferably between about one-half and two and one-half times, the volume of low penetrating fluid is injected. The solvents employed for this purpose may be selected from a wide range of compounds such as, for example, halogenated hydrocarbons, carbon tetrachloride, carbon bisulfide and the like.

When a gel or soap solution has been used for the fracturmg fluid, a peptizer or gel breaker may be incorporated in the viscous liquidand become effective after a time delay. For example, from about one to about three volume per cent of water which becomes effective several hours after the viscous liquid has reached the fracture may be incorporated therein as, for example, by emulsification or the like. Among suitable materials for breaking the gel and reducing viscosity of soap-hydrocarbon gels, it has been found that the water-soluble amines, such as ethanolamine or similar ammoniacal compounds, and the oilsoluble sulfonates are particularly effective. Generally, about one per cent gel breaker based upon a volume of fracturing liquid diluted with a suitable solvent, such as gasoline, is pumped into the drill hole following the fracturing liquid to reduce the viscosity or break the gel so that the latter may be readily burned out of or blown from the fracture. Frequently, where the soap content of the gel is high, concentrations of gel breaker in excess of one per cent are desirable.

What we claim is:

1. In a method for the underground gasification of a combustible carbonaceous deposit the steps which comprise employing a plurality of product gas withdrawal wells extending into said deposit and circularly spaced about a gas processing plant, drilling a plurality of gas injection wells likewise extending into said deposit and disposed about said processing plant in a plurality of circular patterns, said injection wells being located further away from said plant than said product gas withdrawal wells, forming intercommunicating gas permeable channels connecting said withdrawal wells with the circular group of injection wells nearest said withdrawal wells, thereafter igniting the carbonaceous material adjacent said group of injection wells nearest said withdrawal wells, injecting oxygen into the resulting burning zone, taking from said withdrawal wells product gas thus formed in said zone, continuing this operation until the combined quantity of carbon monoxide and hydrogen in said product gas falls below about 70 per cent (dry basis), forming intercommunicating gas permeable channels connecting the next group of injection wells with the area thus burned, igniting the carbonaceous material adjacent said next group of injection wells, thereafter injecting oxygen into said last-mentioned injection wells, conducting the hot product gas thus formed through a previously burned area between said next group of injection wells and said withdrawal wells whereby any unburned carbonaceous material in said area is subjected to further combustion, and recovering product gas from said gas withdrawal wells.

2. In a method for the underground gasification of a combustible carbonaceous deposit the steps which comprise employing a plurality of product gas withdrawal wells extending into said deposit and spaced about a centrally located gas processing plant, drilling a plurality of gas injection wells likewise extending into said deposit and disposed about said processing plant in a plurality of concentrically spaced groups, said injection wells being located further away from said plant than said product gas withdrawal wells, forming intercommunicating gas permeable channels connecting said withdrawal wells with the circular group of injection wells nearest said withdrawalawells, thereafter igniting the carbonaceous material adjacent saidgroup of injection wells, injecting oxy-' gen .into the resulting burning zone in the presence of steam, taking from said withdrawal wells product-gas thus formed in said zone, continuing this operation until the combined quantity of carbon monoxide and hydrogen in said product gas falls below about 70 per cent (dry basis), forming intercommunicatinggas permeable-channels connecting the next group of injection wells with the area thus burned, igniting the carbonaceous material adjacent said next group of injection wells, thereafter injecting a mixture of oxygen and steam into said last-mentioned group of injection wells, conducting the hot product gas thus formed through a previously burned area between said next group of injection wells and said withdrawal wells whereby any unburned carbonaceous material in said area is subjected to further combustion, and recovering product gas from said withdrawal wells.

3. The process of claim 2 in .which the steam-tooxygen ratio is in the neighborhood of from about 1.5 to about 3.0.

4. In a method for the underground gasification of a combustible carbonaceous deposit the steps which comprise employing a plurality of gas withdrawal wellsextending into said deposit and spaced about a centrally located gas processing plant, drilling a plurality of gas injection wells likewise extending into said deposit and disposed about said processing plant in a plurality of concentrically spaced groups in a manner such that the group of said injection wells nearest said gas withdrawal wells is not substantially in excess of about 200 feet therefrom, said injection wells being located further away from said plant than said product gas withdrawal wells, forming intercommunicating gas permeable channels connecting said withdrawal wells with the group of injection wells nearest said withdrawal wells, thereafter igniting the carbonaceous material adjacent the group of injection wells nearest said withdrawal wells, injecting oxygen into the resulting burning zone in the presence of steam, taking from said withdrawal wells product gas thus formed in said Zone, continuing this operation until the combined quantity of carbon monoxide and hydrogen in said product gas falls below about 70 per cent (dry basis), forming intercommunicating gas permeable channels connecting the next group of injection wells with the area thus burned, igniting the carbonaceous material adjacent said next group of injection wells, thereafter injecting oxygen into said last-mentioned injection wells in the presence of steam, conducting the hot product gas thus formed through a previously burned area between said next group of injection wells and said withdrawal wells whereby any unburned carbonaceous material in said area is subjected to further combustion, and recovering product gas from said withdrawal wells.

5. In a method for the underground gasification of a combustible carbonaceous deposit the steps which comprise employing a plurality of product gas withdrawal wells extending into said deposit and disposed adjacent the supporting area of a centrally located gas processing plant, drilling a plurality of gas injection wells likewise extending into said deposit and disposed about said plant in a plurality of circular patterns, said injection wells being located farther away from said plant than said product gas withdrawal wells and circularly disposed thereabout, forming intercommunicating gas permeable channels connecting said withdrawal wells with the group of injection wells nearest said withdrawal wells, thereafter igniting the carbonaceous material adjacent said group of injection wells nearest said withdrawal wells, injecting oxygen into the resulting burning zone, taking from said withdrawal wells product gas thus formed in said zone, continuing this operation until the combined quantity of carbon monoxide and hydrogen in said product gas falls below about 70 percent (dry basis), forming intercommunicating gas permeable channels con:

necting the next group of injection "wellsuwithz-the;area;

thus burned, igniting the carbonaceous rmiaterialadjacent' said next group of injection wells, thereafterrinjecting oxygen into said last-mentioned in ection .wells, conducting the hot product gas thus formed througha previously 1 burned-area between-saidgnext group of'injection wells and-said. withdrawal wells whereby any unburned carbonaceous material in said area is subjected to further.- combustion, and recovering product gas from said gaswithdrawal wells.

6. In a method for the underground gasification of a combustible carbonaceous deposit the steps which comprise employing a plurality of product gas withdrawal wells-extending into said deposit and circularly spaced" culargroup of injection wells nearest said withdrawal.

wells, thereafter igniting the carbonaceous material .adjacent said group ,of injectionwells nearest said withdrawal wells, injecting air into the resulting burningzone,

taking from. said withdrawal wells product gas thus formed in said zone, forming intercommunicating gas permeable channels connecting the next group of injection wells with the area thus burned, igniting the carbonaceous material adjacent said next groupof injection wells, thereafter injecting an oxygen-bearing gas into'said last-mentioned injection wells, conducting the hot product a gas thus formed through a previously burned area between said next group of injection wells and said withdrawal wells whereby any unburned carbonaceous mate: rial in said area is subjected to furthercombustion, and recovering product gasfrom said gas Withdrawal wells.

7. In a method for the underground gasification of a combustible carbonaceous deposit the steps which con-r prise employing a plurality of product gas withdrawal wells extending into said deposit and spaced about a centrally located gas processing plant, drilling a plurality of gas injection wells likewise extending into said deposit and disposed about said processing plant in a plurality of concentrically spaced groups, said injection wells being located further away from said plant than said product gas withdrawal wells, forming intercommunicating gas permeable channels connecting said withdrawal wells with the group of injection wells nearest said withdrawal wells, thereafter igniting the carbonaceous material adjacent said group of injection wells, injecting into the resulting burning zone an oxygen-bearing gas, taking from said withdrawal wells product gas thus formed in said zone, forming intercommunicating gas permeable channels connecting the next group of injection wells with the area thus burned, igniting the carbonaceous material adjacent said next group of injection wells, thereafter injecting an oxygen-bearing gas into said last-mentioned group of injection wells, conducting the hot. product gas thus formed through a previously burned area between said next group of injection wells and said withdrawal wells whereby any unburned carbonaceous material in said area is subjected to further combustion, and recovering productgas from said withdrawal wells.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 947,608 Betts Jan. 25, 1910 1,422,204 1 Hoover et a1 July 11, 1922 2,596,843 Farris May 13, 1952 FOREIGN PATENTS Number Country Date 200,423 Germany July 17, 1908 296,813 Great Britain Sept. 10, 1928 523,333 France Apr. 19, 1921 OTHER REFERENCES Bureau of Mines Report of Investigations-R. I. 4164, August 1947, especially page 33.

Electrical Coal Gasification from Mining Congress Journal, October 1949, page 57. 

1. IN A METHOD FOR THE UNDERGROUND GASIFICATION OF A COMBUSTIBLE CARBONACEOUS DEPOSIT THE STEPS WHICH COMPRISES EMPLOYING A PLURALITY OF PRODUCT GAS WITHDRAWAL WELLS EXTENDING INTO SAID DEPOSIT AND CIRCULARLY SPACED ABOUT A GAS PROCESSING PLANT, DRILLING A PLURALITY OF GAS INJECTION WELLS LIKEWISE EXTENDING INTO SAID DEPOSIT AND DISPOSED ABOUT SAID PROCESSING PLANT IN A PLURALITY OF CIRCULAR PATTERNS, SAID INJECTION WELLS BEING LOCATED FURTHER AWAY FROM SAID PLANT THAN SAID PRODUCT GAS WITHDRAWAL WELLS, FORMING INTERCOMMUNICATING GAS PERMEABLE CHANNELS CONNECTING SAID WITHDRAWAL WELLS WITH THE CIRCULAR GROUP OF INJECTION WELLS NEAREST SAID WITHDRAWAL WELLS, THEREAFTER IGNITING THE CARBONACEOUS MATERIAL ADJACENT SAID GROUP OF INJECTION WELLS NEAREST SAID WITHDRAWAL WELLS, INJECTING OXYGEN INTO THE RESULTING BURNING ZONE, TAKING FROM SAID WITHDRAWAL WELLS PRODUCT GAS THUS FORMED IN SAID ZONE, CONTINUING THIS OPERATION UNTIL THE COMBINED QUANTITY OF CARBON MONOXIDE AND HYDROGEN IN SAID PRODUCT GAS FALLS BELOW ABOUT 70 PER CENT (DRY BASIS), FORMING INTERCOMMUNICATING GAS PERMEABLE CHANNELS CONNECTING THE NEXT GROUP OF INJECTION WELLS WITH THE AREA THUS BURNED, IGNITING THE CARBONACEOUS MATERIAL ADJACENT SAID NEXT 